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Bones Of Contention

Introduction. This item needs an editorial because it shows how a very tiny and obscure brick in the Temple of Science, long thought to be structurally sound, might lead to widespread collapse. Are there other sleepers?

A group of burrowing lizards (the amphisbaenians) possess a heavy bone in their heads that helps them ram their ways through the soil. In surface-living lizards this structure is merely a soft, flimsy cartilage. It was long assumed that the bone in the burrowing lizard developed from the cartilage of its surface-living kin. But a study of embryos now shows that the head bone of the burrowing lizard actually developed from a membrane instead of cartilage. The two similarly located structures are not homologous after all. They had different origins.

Superficially this doesn't seem very anomalous and especially not very exciting. But vertebrate evolution in particular has been charted on the basis of homologous structures. If these structures have different biological origins --
even in just some cases -- the evolutionary family trees may be drawn wrong.

Comment. No one yet knows how serious this problem really is. Basically it means that some animals that look alike (at least bonewise) need not be closely related. To use an analogy, if nature has the plans for a house stored in genetic material, it may be able to build that house out of wood, brick, or what ever material is available.